This invention relates to a magnet design for magnetic resonance imaging and, more specifically, to human body imaging.
For magnetic resonance imaging, a strong and substantially homogeneous magnetic field of 0.1 to 1.5 Tesla is required. Stronger magnets though more expensive are generally considered superior. Magnets with field strengths up to 4 Tesla are being experimented with. Homogeneous indicates that the Z component of the magnetic induction field B is substantially the same everywhere within the imaging volume. A typical imaging volume is 50 centimeter diameter spherical volume. However, specialty imaging magnets are now being considered with a homogeneous volume as low as 15 centimeter diameter spherical volume. A typical homogeneity requirement for the magnetic field is less than 10 parts per million.
Axial design and symmetrical coil pair magnets meeting this requirement are known in the art. U.S. Pat. No. 4,701,736 to McDougall et al. describes axial magnets while U.S. Pat. No. 4,398,150 to Barjhoux et al. describes symmetrical coil pair magnets.
Superconducting magnets of conventional design, used in whole body scanners, require the patient to be inserted in a long hollow bore in the center of the magnet. Patient encapsulation, cost and size of the magnet are significant problems with the prior art. So-called planar magnet designs have been conceived, for example, U.S. Pat. No. 4,701,736 to McDougall, describes such a magnet design, but are quite expensive in that they use a large amount of superconducting wire.
In prior art magnetic resonance imaging magnets, homogeneity has been achieved by providing a small number of very large superconducting coils in low temperature cryogenic vessels with extremely limited heat loss. The advent of high temperature superconductors, particularly yttrium-barrium2-copper3-oxygen7, and cryocoolers allows alternative magnet structures to be considered. A cryocooler is a device for removing heat from a cryogenic, fluid such as liquid helium or liquid nitrogen.
There is a broad demand for special purpose magnetic resonance imaging systems that can be placed in doctors' offices and small clinics. However, the cost size, weight and fringe field of the magnet component of currently available magnetic resonance imaging systems have precluded this.